FileSystem: make all classes final to fix Sendable errors

[MaxDesiatov]

These classes can't be Sendable if they aren't final. Somehow it didn't cause CI failures before, but had impact on Swift Driver build jobs.

rdar://108040704

[MaxDesiatov]

@swift-ci test

[tomerd]

I wonder if all these need to be classes. do they carry any state?

[neonichu]

I wonder if all these need to be classes. do they carry any state?

I think local filesystem doesn't since it is just a wrapper around other APIs, but the various virtual-ish filesystems need to.

[MaxDesiatov]

InMemoryFileSystem does have its own state, which itself is a class, and it probably would be weird if assigning it to a different variable and modifying it created a copy, as underlying storage is CoW. It could probably even break some existing behavior. RerootedFileSystemView on the other hand only holds a reference to an underlying filesystem, so from that perspective it's an aggregate and could be a struct. If you'd like any relevant changes to be made, should they go to this same PR or a separate one?

[MaxDesiatov]

Enabling auto-merge just for the sake of unblocking whatever CI job uncovered this as an error in Swift Driver.

[tomerd]

right, InMemoryFileSystem makes sense to be a class given that it has state. the point I was trying to make is that we should audit them and make sure we only use class where we need to and change the others to structs. this PR is fine IMO

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test windows